Photodynamic therapy using porphyrins and related compounds has been known in the art for some time. As early as the 1940's, it was known that porphyrin had the capability of fluorescing in tumor tissue. The porphyrins appear to localize in tumor tissue where they absorb light at certain wavelengths when irradiated, providing a means to detect the tumor by the location of the fluorescence. Accordingly, preparations containing the porphyrins are useful in the diagnosis and detection of such tumor tissues. In addition, the porphyrin compounds also have the capability of destroying the tumor tissue when irradiated at the appropriate wavelength, possibly through the formation of singlet oxygen. (Weishaupt, K. R., et al., Cancer Research (1976) pp. 2326-2329).
The use of these light absorbing compounds, particularly those related to porphyrins, has been well established as a treatment for tumors when administered systemically. The utility of the compounds rests upon their ability to localize in neoplastic tissue while being cleared from the normal surrounding tissue. (See, for example, Dougherty, T. J. et al., "Cancer: Principles and Practice of Oncology" (1982). V. T. de Vita, Jr. et al., eds., pp. 1836-1844).
In addition to systemic use for the treatment of tumors, more recent publications have specified alternative uses for the porphyrin compounds. For example, the use of porphyrins in the treatment of skin diseases has been described in U.S. Pat. No. 4,753,958. The use of photosensitizing compounds to sterilize biological samples containing infectious organisms such as bacteria and virus has been disclosed in U.S. Pat. No. 4,727,027 where the photosensitizer is furocumarin and its derivatives. Photosensitizing porphyrins are useful in the detection and treatment of atherosclerotic plaques, as described in U.S. Pat. Nos. 4,512,762 and 4,574,682. In addition, U.S. Pat. Nos. 4,500,507 and 4,485,806 describe the use of radiolabeled porphyrin compounds for tumor imaging.
A photosensitizer preparation widely used in the early stages of photodynamic therapy both for detection and treatment was a crude derivative of hematoporphyrin, also called hematoporphyrin derivative, HpD, or Lipson derivative, prepared as described by Lipson et al., in J. Natl. Cancer Inst. (1961) 26: 1-8. Considerable work has been done using this preparation and the use of this derivative in treatment of malignancy has been widely reported. (Cancer Res. (1978) 38:2628-2635; J. Natl. Cancer Inst. (1979) 62:231-237).
Dougherty and coworkers prepared a more effective form of the hematoporphyrin derivative which is prepared by ultrafiltration of HpD to reduce the content of low molecular weight species. This work is the subject of U.S. Pat. No. 4,649,151, hereby incorporated by reference into the present application, which further describes in detail methods for phototherapeutic treatment of a patient using the compositions described therein. This form of the drug is actually a complex mixture containing porphyrin units joined by ether linkages (Dougherty, T. J. et al., Adv. Exp. Med. Biol. (1983) 160:3-13) and ester linkages (Kessel, D. et al., Photochem. Photobiol. (1987) 36:463-568). This complex mixture, referred to herein as polyhematoporphyrin ethers/esters ("PHE"), has been available under the trademark PHOTOFRIN II.RTM., and is the subject of the present invention.
At present, PHE has been supplied as a 2.5 mg/ml or 5 mg/ml solution containing PHE in normal saline. The PHE degrades rapidly when exposed to heat and is therefore relatively unstable at room temperature. Accordingly, the solution must be kept frozen to maintain its potency. In addition, the solution tends to show significant particulate formation at higher temperatures which makes it undesirable for use as an injectable product unless it is kept frozen and thawed immediately prior to use.
The use of such a frozen solution has many disadvantages, however. Because it has to be kept frozen, it must be shipped and stored in a frozen state, necessitating the use of special refrigeration conditions. For example, the product must be shipped in special containers using dry ice or the like as a refrigerant. This is a major drawback, adding to the cost and logistics of using the product. At the point of use, the frozen solution must be stored at -20.degree. C., which is below the operating temperatures of some freezers thereby necessitating special freezer equipment. In addition, the frozen product must undergo a thawing period and is therefore not useable immediately with a patient.
Thus, there is a need for a formulation of PHE which is stable at room temperature for extended periods of time, does not have to be kept frozen and therefore does not require special shipping and storage conditions.
It is known in the art that freeze-drying a product which is relatively unstable in aqueous solution can result in a product that is stabilized and therefore has a longer shelf life than an aqueous solution. Additionally, a freeze-dried product has an advantage over a product in powder form in that it is rapidly soluble and easily reconstituted prior to administration by injection. Another advantage of freeze-drying a product unstable in aqueous solution is that it can be processed and filled into dosage containers in a liquid state, dried at low temperatures thereby eliminating adverse thermal effects, and stored in the dry state where it may be more stable. (See Remington's Pharmaceutical Sciences, 15th edition., pp. 1483-1485 (1975)). Thus, freeze-drying would be an ideal method of obtaining a formulation of PHE which would have the desired stability at room temperature and therefore would not have to be stored frozen.
Weinstein et al., in U.S. Pat. No. 4,753,958 discloses a topical formulation of hematoporphyrin derivative for the treatment of psoriasis and other cutaneous diseases which is prepared by freeze-drying a 5 mg/ml saline solution of PHE and reconstituting it in an appropriate topical vehicle.
Although freeze-drying the frozen PHE saline solution may be useful for the purpose of preparing a product suitable for topical application, the present inventors have found that certain problems are encountered when freeze-drying the saline solution which make formulation difficult and the resulting product unacceptable for administration by injection. When attempts were made to freeze-dry a concentrated saline solution containing 15 mg/ml PHE and 5.4% sodium chloride (the amount of sodium chloride necessary to yield an isotonic solution after reconstitution with Water For Injection for a 2.5 mg/ml PHE solution), partial precipitation (salting out) of the PHE active ingredient occurred. In addition, as a consequence of the precipitation, filtration of the concentrate is extremely difficult, requiring frequent filter changes. Also, some of the active ingredient is removed on the filters. Further, the freeze-dried product is not homogeneous and consists of separate sodium chloride and PHE phases. Separation of the phases probably takes place during freezing due to differential crystallization.
Alternatively, a saline solution containing 2.5 mg/ml PHE could be freeze-dried, but the freeze-drying process would be very long, since more water would have to be removed. In addition, the same problems with filtration and precipitation would be encountered due to the presence of sodium chloride in the solution.
There is a clear need, therefore, to provide a formulation of PHE which is stable at room temperature over an extended period of time and therefore does not require freezing, but which overcomes the problems associated with freeze-drying the aqueous saline solution.